tcg/mips: Support bswap flags in tcg_out_bswap16

Merge tcg_out_bswap16 and tcg_out_bswap16s.  Use the flags
in the internal uses for loads and stores.

Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>